The overall objective of this proposal is to further our understanding of the effects of the transforming function of RNA tumor viruses on the expression of differentiated functions as well as to understand the molecular basis of these effects. To this end, the expression of the myosin heavy chain (MHC) gene in a myoblast cell line and its temperature sensitive (ts) mutants for either growth of differentiation will be studied. These cell lines have already or will be transformed with wild type or ts mutants for transformation of RSV. The expression of the MHC gene at permissive (33 degrees C) and non-permissive (41 degrees C) will be studied on both transformed and non-transformed cells.